1. Field of the Invention
The present invention relates to a vacuum pump, that is, a turbomolecular pump, wherein a plurality of rotor and stator blades which are combined together are rotated relative to each other under a low pressure such that any collision between gas molecules is negligible to effect exhaustion of a gas. The present invention also pertains to a method of operating a vacuum pump of the type described above.
2. Description of the Prior Art
A typical conventional turbomolecular pump will first be explained with reference to FIG. 1.
A conventional turbomolecular pump which is generally denoted by the reference numeral 1 includes a motor 2, a motor shaft 3 for transmitting the rotational force derived from the motor 2, a rotor 4 secured to the motor shaft 3, a plurality of rotor blades 5 fixed to the rotor 4, a plurality of stator blades 6 each disposed between a pair of adjacent rotor blades 5, a spacer 7 having the stator blades 6 attached thereto, a casing 10 provided with a suction port 8 and an exhaust port 9, and a protective net 11 for protecting the rotor and stator blades 5 and 6. In operation, the motor 2 is driven to rotate the rotor blades 5 at high speed in a high-vacuum atmosphere sufficient to ensure that molecular flow is available, thereby sucking gas molecules from the suction port 8, compressing the gas at a high compression ratio and moving the gas toward the exhaust port 9, thus producing a high vacuum.
The above-described conventional turbomolecular pump suffers, however, from the following problems. The gas exhausting performance of the pump depends on the molecular weight of a gas being handled by it. When a gas having a low molecular weight is being handled, the gas exhausting performance deteriorates to a considerable extent. The lower the compression ratio, the lower the gas exhausting performance. The blade speed ratio C, a parameter representing the compression ratio, is expressed as follows: EQU C=V/Vm
(wherein V is the peripheral speed of the rotor blades and Vm is the maximum probability speed of gas molecules).
The maximum probability speed Vm of gas molecules is expressed as follows: ##EQU1## (wherein M is the molecular weight of the gas, K is Boltzmann's constant, and T is the absolute temperature of the gas).
As will be clear from these expressions, the lower the molecular weight M of the gas, the higher the maximum probability speed Vm of the gas molecules and the lower the blade speed ratio C. Therefore, when a gas having a low molecular weight is being handled, the gas exhausting performance is low. Many problems are likely to occur in actual operation of the turbomolecular pump when the gas exhausting performance is low.
Among the problems associated with gases having low molecular weights, the existence of water vapor, in particular, adversely affects the gas exhausting performance of the pump. In a system wherein a part of the system that is provided with a turbomolecular pump is open to the atmosphere and air flows into the system, the greater part of the residual gas under a vacuum of about 10.sup.-4 Torr to 10.sup.-10 Torr (10.sup.-4 mmHg to 10.sup.-10 mmHg) which is produced by the turbomolecular pump is water vapor. The residual water vapor has adverse effects on the degree of vacuum and the vacuum environment.
In the case of using a cryo-vacuum pump that employs a helium refrigerator and a heat exchanger which provides ultra-low temperatures of from about 15.degree. K. to about 20.degree. K, the gas exhausting characteristics in regard to water vapor are improved and it is therefore possible to cope with the above-described problems to a certain extent. However, such a cryo-vacuum pump involves the following problems:
(1) Since a refrigerator for ultra-low temperatures is used, it takes a long time to start and suspend the system. PA1 (2) Since the pump is a capture type one, i.e. it freezes and traps most gas molecules, it must be regenerated for a long period every time a predetermined load is run and completed. PA1 (3) Since the sublimation temperature differs depending upon the kind of gas molecules, various kinds of gas molecules are separated from each other and successively discharged from the pump at high concentrations as the temperature of the heat exchanger rises during a regenerative operation, and it is difficult to treat various kinds of gases which are discharged separately. In particular, in semiconductor manufacturing processes, toxic, highly-corrosive, explosive and combustible gases, for example, monosilane (SiH.sub.4), hydrogen fluoride (HF), etc., are used that are diluted with inert gases such as nitrogen (N.sub.2), helium (He), etc., and it is therefore extremely difficult to handle these various kinds of gases that are discharged separately.
It might be considered possible to combine the conventional turbomolecular pump and cryo-vacuum pump in order to overcome the above-described problems. However, with such a combination, most gas molecules exclusive of hydrogen and helium molecules would be freeze-trapped in the cryo-vacuum pump and therefore the provision of the turbomolecular pump would become meaningless.